Data acquisition using MRI systems, in the past required that the data acquisition be substantially completed prior to image reconstruction. In the reconstruction of the images the data is generally transformed using two-dimensional Fourier transforms. The data acquisition scan in the past proceded from an edge of a two-dimensional Fourier transform plane through the center of the plane to an opposite edge. Most of the large data points are found around the center of the Fourier transform plane. Thus, it has been necessary for the MRI system to first acquire at least half of the data prior to processing the data to provide the image.
More recently apparatus and methods (see, for example, U.S. Pat. No. 4,721,972) have been promulgated wherein the acquisition proceeds from the center of the Fourier transform plane outward enabling the almost immediate processing of the data to provide the image.
However, experience has shown that acquisition from the center outwards may cause an artifact, including a streak-like artifact emanating from any sharp boundary in the image. Such artifacts stem from the fact that when the most significant data points were acquired (the zero encoding), the system had not yet reached a state of dynamic equilibrium. This may occur because various hardware components take time for each equilibrium, or, more importantly, because the eddy currents generated by the switched gradients take time to reach equilibrium. Thus, it would seem advantageous to acquire the less significant data points at the start, and delay acquisition of the largest data points until we are certain the system has reached its equilibrium state.
There is yet another reason for slightly delaying the acquisition of the zero encoding cycle. When the scan starts, the sudden noise that results from the gradient switching often startles the patient, possibly causing him to move. As before, it would be advantageous to have such patient motion occur during acquisition of one of the off center cycles rather than at the zero encoding.
Of course, both of these problems can be overcome by applying "dummy" cycles before commencing actual data acquisition; i.e., running the scan but not storing the data. However, this method is wateful of valuable imaging time.
Data acquisition requires that the subject being imaged be placed in a strong static magnetic field. The static magnetic field aligns the protons of certain elements within the body. The most prevalent element subject to such alignment is hydrogen. The aligned proton are subjected to Rf pulses which nutate the aligned protons when the frequency of the Rf pulses is the Larmor frequency; i.e.: EQU f=Bo.gamma./2.pi.
where:
.gamma. is a constant for each element, PA1 Bo is the strength of the magnetic field at the location of the proton being nutated, and PA1 .pi. is the constant 3.1416+
Subsequent to the application of the Rf pulse, the nutated protons tend to dephase in the transverse plane and also to realign themselves with the magnetic field. The movements of the nutated protons generate what are known as free induction decay (FID) signals. These signals are proportional to the density of the nutated protons and are used as the raw data for reconstructing images. The raw data is transformed using two-dimensional Fourier transforms to obtain data for image pixels which correspond to locations in the subject.
The locations of the FID signals in the subject are obtained in a well known manner by applying gradient fields to the static magnetic field during the data acquisition process. The frequency of the received FID signals are a function of the strength of the magnetic field and the strength of the magnetic field is a function of location in the magnetic field along the X, Y and Z axes; therefore, the locations of the nutated protons are obtained from the frequencies of the received signals.
Three gradients are generally used. The are known as the select gradient Gz, the encoding gradient Gy and the viewing gradient Gx. In effect, each gradient field is used for determining one dimension of the three dimensions needed to locate the protons in a selected volume of the patient.
The encoding gradient is very often a phase gradient, that is the phase accumulated by the spins is varied by changing the amplitude of the gradient pulse at each application of the gradient. For example, in a 256.times.256 image, 256 such encoding gradient pulses of different amplitudes are applied for each of the other two gradients applied. In the prior art, the image reconstruction does not commence until at least half of the encoding pulses have been applied. In the past, in general, the first encoding gradient pulse applied was the maximum negative encoding gradient pulse followed by the maximum negative encoding gradient plus 1 and sequentially through the maximum positive encoding gradient.
As is well known, at the zero value of the encoding gradient, the received signal is generally largest, that is, most of the data is acquired. Thus, image reconstruction could not start until at least the zero encoding gradient pulse was applied i.e. until at least half the gradients were applied. Where only half the encoding gradients were used computations were necessary to construct the other half of the data.
If data reconstruction could be done on-line (that is on-the-fly) while the data acquisition is in process; then, it would be possible to save valuable throughput time. In contrast, following the prior art procedures, substantially all of the data is acquired before the image is constructed. Then, if the physician sees that the image is in the wrong place, for example, the whole process must be repeated and all of the acquisition and reconstruction time is wasted.
Where the image is constructed on the fly, valuable time is saved since the physician sees if the image is in the wrong place before the image is completed and accordingly he can restart the procedure and move the selected slice without having to wait for the entire data acquisition. Also for example, as the data acquisition proceeds the image becomes progressively clearer; thus, the physician often obtains sufficient information prior to the acquisition of all of the data; thereby, saving more throughput time. Accordingly, it would be advantageous to process the data and reconstruct the image during the acquisition of the data. However, efforts to acquire data by starting acquisition from the center of the Fourier plane in the past resulted in serious artifacts.
Accordingly, it is an object of the present invention to provide methods for acquiring useable relatively artifact-free images on the fly in MRI systems.